The need for data transmission between network nodes as well as the quantity of data transmitted between network nodes continues to increase with a very high rate of growth. The growth is based, on the one hand, on the increasing number of network nodes such as, for example, fixed installed or mobile computer systems. On the other hand, the growth is based on the larger and larger quantities of data, such as in particular multimedia data. Such data comprise e.g. digital data such as texts, graphics, pictures, animations, sound recordings or video recordings. Belonging thereto are in particular also sound and video data, according to the MPx (e.g. MP3) or MPEGx (e.g. MPEG7) standards, for example, as they are defined by the Moving Picture Experts Group (MPEG), or picture data such as, for instance, according to the PNG standard (PNG: Portable Network Graphics), which is defined in the RFC 2083 (RFC: Request for Comments) of the IETF (Internet Engineering Task Force). Thus, for example, by means of a digital photo camera, first picture data are captured by a first network node, shown, and, if need be, processed. Then these picture data or parts of these picture data are transmitted via a communication link to a second network node, the picture data being stored, shown and if need be further processed on the second network node. Used as communication networks for transmission of data are e.g. a LAN (Local Area Network), a WAN (Wide Area Network) or the Internet via e.g. a public switched telephone network (PSTN: Public Switched Telephone Network), via a mobile radio network (PLMN: Public Land Mobile Network) such as e.g. a GSM or a UMTS network (GSM: Global System for Mobile Communication, UMTS: Universal Mobile Telephone System) or via a WLAN (Wireless Local Area Network).
In the last few years, mobility, and with it, the number of mobile network users has likewise increased sharply, in particular the number of mobile Internet users. The growing offer in IP-capable (IP: Internet Protocol) mobile devices, such as e.g. PDAs (PDA: Personal Digital Assistant), mobile radio telephones or laptops, goes hand in hand with this development. The transition from fixed network nodes to more flexible demands through increased mobility has only just begun. In mobile radio telephony, for example, this tendency also shows itself in, among other things, new standards such as GPRS (General Packet Radio Services), EDGE (Enhanced Data GSM Environment), UMTS (Universal Mobile Telecommunications Service), HSDPA (High-Speed Downlink Packet Access) or HSUPA (High-Speed Uplink Packet Access). Mobile computer use distinguishes itself in many ways from computer use and network capability in fixed networks. For mobile network use, a mobile network node has today at its disposal various network interfaces for connection of the mobile network node to different communication networks. The different network interfaces thereby also have different performance features such as, for example, different data transmission rates, bit error rates, availabilities, or a different energy consumption. Thus, for instance, a GSM based mobile radio network is characterized in particular by a high, location-independent availability, the network interface for connection to a GSM based mobile radio network has a relatively minimal data transmission rate and a relatively minimal energy consumption. In contrast, the connection via a WLAN network interface is only possible at locations with corresponding WLAN access points, the WLAN network interface having a relatively high data transmission rate and a relatively high energy consumption. Finally, a UMTS network is characterized, for example, by a more and more location-independent availability, the UMTS network interface having a relatively high data transmission rate and a relatively high energy consumption. The available network interfaces of a network node are often activated at the same time independently of an actual data transmission. This leads to too high an energy consumption, however, which is undesirable, in particular for locations without electrical power socket for the energy supply of the mobile network node. However, the mobile network nodes normally have means to switch off a network interface manually or automatically, for example if a monitoring module detects that a network interface remains unused for a certain interval of time. A manual switching off has the drawback that the user has to activate the network interface again manually before a data transmission. For an automated data transmission, such as, for example, for the automated download of e-mail messages by an e-mail client of the mobile network nodes, it is difficult, however, to active a corresponding network interface at the correct points in time manually, and, if need be, deactivate it again. On the other hand, with an automatic switching off of a network interface, this network interface is typically also automatically activated again, as soon as an application of the network node attempts to access a network or a second network node. This has the drawback, however, that with an accessing of an application of the network node on a central unit, the network interface is often unnecessarily activated, such as, for example, when accessing an e-mail server by an e-mail client of the mobile network node to check the status of the e-mail account. A further disadvantage of the state of the art is that a deactivated network interface, which is configured, for example, for receiving data that require a high data transmission rate, is no longer accessible from a second network node. Thus, for example, for a video telephony application of a second network node, it is no longer possible to accomplish a call to a mobile network node with a deactivated network interface, but one configured however for receiving a video telephony call.